As is also known in the art, liquid crystals (LC's) cells are typically formed by mating together two plates of glass (or other suitable material) and sealing the edges to form a cavity between the two glass plates. One piece of glass is formed or otherwise provided having a so-called fill hole provided therein. Once the cavity is formed, liquid crystal material is introduced into the cavity through the fill hole. After filling the cavity, the fill hole is closed.
In some applications (e.g. space-based applications), a hermitically sealed LC cell is required. Thus, for such applications, a hermetic seal and associated manufacturing process is needed that does not reduce the LC performance or compromise reliability.
There are many ways to form hermetic edge seals around the cell before the LC is introduced. Glass frit has been used to “weld” two glass panels together to form the cavity into which liquid crystal material is introduced through the fill hole. However, the fill hole still remains open and the temperatures needed to melt the frit and thus close the fill hole are too high for the LC material introduced into the cavity. Soldering can also be used to provide a hermetic edge seal, but also does not solve the fill hole problem.
U.S. Pat. No. 4,135,789 describes sandwiching glass beads between a Teflon plug in the fill hole and a metal cover. In this approach, the cover is heated by induction to melt the glass beads and the Teflon plug protects the LC.
U.S. Pat. No. 4,418,102 describes how epoxy can be used to close the fill hole. There are at least three problems with this approach for the application of high performance LC cells (e.g. LC cells which may be used in space-based applications). The first problem is that epoxies do not create true hermetic seals and can provide leakage paths in a vacuum environment. The second problem is the fact that epoxies out gas. Both of these failure mechanisms are fundamental issues in space-based applications. The third problem is long-term interaction between the epoxy and the liquid crystal. High performance LC's used in optical phased array (OPA) technology, for example, has shown detrimental interaction with standard LC epoxies.
Thus, to use high performance liquid crystals (LC's) in a space environment, a hermetic seal and associated manufacturing process is needed that does not reduce the LC performance or compromise reliability. Conventional techniques for sealing LC cells and panels involves either epoxies or plug techniques both of which have been found incompatible with some high performance LC's used in optical phased array (OPA) technology.